1. Field of the Invention
The present invention relates to a drug ejection apparatus which is portable and is applicable to an inhalation apparatus for inhalation of a drug by a user. The present invention relates also to a method for estimating the ejection performance of the drug ejection apparatus.
2. Related Background Art
Inhalation apparatuses have been developed which allow a user to inhale, through a mouthpiece, fine liquid drug droplets ejected based on the principle of an inkjet system (US Patent Application Publication No. 2007-0227534). Such an inhalation apparatus is capable of spraying precisely a prescribed amount of the drug in a uniform particle diameter.
Such a drug ejection apparatus is constituted basically of an ejection head having an ejection energy-generating element like a heating element and a drug tank for holding a drug to be fed to the ejection head. For efficient absorption of the ejected liquid drug by a lung of a user, the liquid droplets should have a fine diameter, as fine as several microns, and for the ejection of the fine liquid droplets, the orifice of the ejection head should have an orifice diameter of several microns.
Owing to the extremely fine diameter of the orifice, when one and the same ejection head is used for several times of inhalation, the liquid drug remaining after the preceding drug ejection can dry to become viscous and can stick to the inside wall of some of the orifices (nozzles) to clog the orifice. With repetition of the use of the ejection head, the number of the orifices available for the subsequent drug ejection can be decreased, whereby the amount of the drug ejected for the inhalation can become less than the intended amount even when the head is driven under the same conditions for the same period.
Therefore, when the drug is ejected under fixed driving conditions by assuming that the ejection can be constant under the fixed conditions, the intended amount (prescribed by a medical doctor) of the drug can not be ejected.
Desirably, the amount of the drug ejected through the ejection head is measured precisely in-situ, and the ejection is stopped on ejection of the intended amount of the drug. However, this is impossible practically.
Otherwise, the clogged nozzle can be recovered by suction with a pump in a manner as conducted in ink-jet printers. However, this is not suitable for the inhalation apparatus, since the inhalation apparatus, which is carried always by the user, should be small and lightweight and the suction mechanism like a pump is not preferred, and the drug is generally more expensive than the ink and discard of the drug by suction is undesirable.